1 You know that most mass movements are caused by gravity. Water is another powerful force that can cause the movement of large masses of earth. Movement of rocks and soil can be caused by too much water or too little.2 If there is too little water on a slope, particles have trouble holding together. It's sort of like trying to make a snowball out of dry, powdery snow. It just won't keep its shape. Wet snow stays together easily. The same is true of soil. Water helps hold the grains together. If the grains are dry they can be moved easily by wind or gravity.3 Sometimes there is too much water on a slope. The particles become loose. Water adds to the weight of the earth materials. It also reduces the friction of the surfaces. It is easier for gravity to pull the soil and rocks downhill.